Effects of impact fatigue on residual static strength of adhesively bonded joints

In most industrial applications, adhesive joints experience impact fatigue loads in service. The energy of each impact is too low to cause joint failure. Although the repetitive impacts usually do not apparently affect the joints, they can significantly reduce the strength of adhesively bonded structures. Accordingly, understanding the effect of impact fatigue on the residual strength of the bonded components is crucial in real applications. This study deals with this issue where the effect of impact fatigue on the residual static strength of single lap joints is analyzed. To achieve this, the manufactured single lap joints were categorized into four different groups. Group 1 joints were tested under static loading conditions. Joints in group 2 were tested under impact to obtain the impact strength of the single lap joints. To analyze the impact fatigue life of the joints, the single lap joints in group 3 were tested under cyclic impact at different energy levels until failure. To investigate the effect of impact fatigue on the residual static strength of the joints, single lap joints in group 4 were tested under a specific number of impact cycles followed by a static tensile test. Using microscopic analysis, the fracture surfaces of the tested specimens were analyzed. The results showed that cracks initiate from the middle of the bonded area as a result of cyclic impact stress waves. Then, by increasing the number of impacts, a large number of cracks nucleate from the edges of the joints and grow along the width to the middle of the overlap. A 3D finite element method was employed to analyze the stress distribution along the bondline under impact loads.